In recent years, opportunities to use an electrophotographic copier and a printer have been increased in the field of printing as well as color printing. In the field of printing as well as color printing, high quality digital monochromatic or color images tend to be demanded. In order to respond to such the demand, it is proposed that a laser light having a short wavelength is employed as a source for exposure to light to form high definition digital images. However, the electrophotographic image finally obtained has not sufficiently achieved high image quality, even though the laser light having a short wavelength is employed, and the dot size of exposure is narrowed to form a minute electrostatic latent image on the electrophotographic photoreceptor.
The reason is that photosensitive properties of the electrophotographic photoreceptor, an electrification characteristic of toner in a developer and so forth do not satisfy properties desired for formation of minute dot latent images as well as formation of toner images.
That is, in cases where the electrophotographic photoreceptor is an organic photoreceptor developed for a conventional long wavelength laser, (hereinafter, also referred to simply as a photoreceptor), reproducibility of dot images tends to be degraded since a sensitivity characteristic is degraded, and no clear dot latent image is formed, when imagewise exposure in which the dot size of exposure is narrowed is conducted with laser light having a short wavelength.
Anthanthrone based pigments and pyranthrone based pigments are conventionally well known as a charge generation material in a photoreceptor utilized for a short wavelength laser (Patent Document 1). However, there is no description in Patent Document 1 concerning polycyclic quinone pigments such as the anthanthrone based pigments subjected to a specific treatment, and they are simply considered to be commercially available pigments, but as for properties such as sensitivity and so forth obtained when these commercially available pigments are employed, neither sensitivity nor high speed performance is sufficiently obtained with a high speed printer and copier equipped with a short wavelength laser which is expected to be developed in the near future. On the other hand, it is well known that the charge generation material size is minimized to form a charge generation layer having high density of the charge generation material in order to improve sensitivity. However, when this particle-minimizing technique is applied to a photoreceptor suitable for a short wavelength laser, sensitivity itself is improved, but image defects caused by generation of memories and very small charge leakage via repetitive electrification in a charging step and a transfer step during image formation tend to be generated.
(Patent Document 1) Japanese Patent O.P.I. Publication No. 2000-47408